It's amazing how many people do not know how open rates are calculated. Loren McDonald rightfully is trying to convince the email marketing world to change the name from "open rate" to "render rate" because that is what it is. Read his articles and the comments on them here and here.

Open rates are tracked by the number of times a certain image was downloaded from the sending server. So if images are not being downloaded it will not be able to track whether or not you've opened the email.

Let me explain with two examples:

Let's say your email client downloads images by default and you use a preview pane to scroll through your inbox. In this case when you preview an email in the preview pane, even if it's only for half a second, this will be counted as an open.

If your email client blocks images by default and you open an email and read it without downloading the images, an open will not be tracked.

In the first case, an open is tracked even though you didn't read the email, in the second case you read the email but an open is not tracked because the images were not downloaded. Only when you view an email (it doesn't matter if you open it or view it in the preview pane) and download the images, an open will be tracked.

Keep this in mind when you evaluate your open rates. The only reason you should look at open rates over time is to determine a trend. Don't use the open rate as an indication of how many people actually opened and read the email, because you might be way off.